The Bush administration said Tuesday it has imposed sanctions on nine foreign companies for selling Iran hardware or ingredients for weapons of mass destruction. Those penalized include six Chinese firms, two from India, and one from Austria.

The sanctions action was one of the most sweeping yet taken under the Iran Nonproliferation Act approved by Congress five years ago, and it reflects an aggressive behind-the-scenes U.S. effort to cut off Iran from sources of weapons of mass destruction material.

The State Department said the year-long sanctions against the nine companies went into effect December 23, and will bar them from doing business with the United States government or from buying high-tech U.S. equipment and products.

Six of the firms to face the U.S. sanctions are Chinese and include three companies sanctioned in the past for violating the 2000 act of Congress.

One of them, China North Industries Group, known as NORINCO, has been cited repeatedly under the law and was described by one senior official here as a serial proliferator.

Both Indian companies cited are chemical exporters. The Austrian firm, Steyr-Mannlicher, is a well-known maker of advanced assault weapons.

In a talk with reporters, State Department Deputy Spokesman Adam Ereli said all nine firms are being sanctioned based on credible information that they transferred to Iran equipment or technology on an export blacklist.

He defended the sanctions even though they will have only symbolic meaning for several of the named companies, which have no known dealings with the United States. He said the listing will expose the firms to scrutiny by their home governments as well as international community:

"It's an important and effective tool in constraining Iran's efforts to develop missile and WMD capabilities," he said. "It does have an impact, I think, particularly in alerting governments to activity taking place in their countries, and instituting measures or talking actions to prevent those kinds of activities."

Mr. Ereli said while the responses of foreign governments to the Iran sanctions have varied, the United States has gotten good cooperation from Austria in the latest case.

He said weapons proliferation is a slippery business and that there are always going to be those who will put narrow commercial interests ahead of the greater good.

More than 40 firms and individuals have been cited under the act since the first sanctions were imposed in 2001.

Many of those entities are Chinese even though Beijing government denies that it or any Chinese companies have supplied missile or weapons-related technology to Iran or other countries of concern.

An unclassified Central Intelligence Agency report to Congress last November said Iran was vigorously pursuing indigenous nuclear and chemical-biological weapons programs, and focusing procurement efforts on commercial entities in Russia, China, North Korea and Europe.

The CIA report credited China with some positive anti-proliferation steps, but said the behavior of Chinese companies remained of great concern.

It said assistance by Chinese entities had helped Iran move toward its goal of self-sufficiency in the production of ballistic missiles.

It also said there was evidence that Chinese firms had provided Iran with so-called dual-use items applicable to chemical weapons.